As is well known, fluorocarbon rubbers in general have excellent properties such as heat resistance against chemical and mechanical strengths so that they are industrially used in a variety of fields including the industries of automobiles and machinery. A problem in the industrial application of fluorocarbon rubbers is that the workability thereof in the milling and molding behavior and releasability from metal molds is relatively poor as compared with other rubbers, not to mention the expensiveness of fluorocarbon rubbers in general.
In addition to the expensiveness of a fluorocarbon rubber per se, shaped and vulcanized articles of a fluorocarbon rubber are expensive as a consequence of the very high costs for the preparation of vulcanized articles from the rubber composition. For example, fluorocarbon rubbers generally have poor milling workability so that an unduly long time is taken for the roll milling work. The molding and vulcanizing works of the fluorocarbon rubber composition must be performed at a relatively high temperature taking a long time. Further, a secondary vulcanization treatment at a high temperature taking a long time is indispensable in order that the shaped and vulcanized rubber articles can exhibit fully stabilized properties. This is the reason for the limited industrial application fields of fluorocarbon rubbers as compared with other elastomers.
Various attempts have of course been undertaken heretofore for the improvement of the workability of fluorocarbon rubbers. For example, Japanese Patent Kokai 3-22308 proposes a method in which a fluorocarbon rubber is compounded with a silicone-based additive having a siloxane linkage so as to impart the fluorocarbon rubber composition with improved workability in roll milling and releasability from metal molds in molding. This method, however, has several problems and disadvantages. For example, troubles are sometimes caused by the bleeding of the silicone-based additive on the surface of the rubber articles as molded due to the limited miscibility of the silicone-based additive compound with the fluorocarbon rubber along with a problem in respect of the storage stability of the composition. Furthermore, a decrease is caused in the mechanical properties of the shaped and vulcanized rubber articles as a consequence of the admixture of a silicone-based additive. Japanese Patent Kokai 60-101135, on the other hand, proposes that a fluorocarbon rubber is blended with other synthetic rubbers or resins with an object to decrease the cost of the rubber composition. Only very few cases have been successful in the industrialization of this method because the rubber articles prepared in this way are poor in properties as compared with those obtained from a fluorocarbon rubber, as such, for the same reasons as in the case of the silicone-based additives.
As to the polyol-crosslinkable fluorocarbon rubber, which is a copolymer of, for example, vinylidene fluoride and hexafluoropropylene crosslinkable with a polyol compound such as bisphenol A, among various types of fluorocarbon rubbers, in particular, a proposal has been made in Japanese Patent Kokai 58-111848 that a polyol-crosslinkable fluorocarbon rubber is stored under a hermetically sealed condition and processing thereof is undertaken in the presence of a drying agent or under dry heating with pressure reduction because the crosslinking reaction of the rubber gradually proceeds in contact with moisture-containing air during storage and processing resulting in a decrease in the properties of the vulcanized rubber article. Alternatively, a solution composition of a polyol-crosslinkable fluorocarbon rubber is proposed by dissolving the rubber in an organic solvent. Although the rubber molecules can be shielded from the atmospheric moisture by the solvent, the crosslinking reaction of the rubber molecules proceeds gradually even by reacting with a trace amount of water contained in the organic solvent finally resulting in gelation of the solution unless a special means is undertaken in order to decrease the water content in the solvent as completely as possible so that the pot life of the working solution can be extended. At any rate, no very effective method is known for the improvement of the storage stability of a polyol-crosslinkable fluorocarbon rubber.